George Santayana was a Spanish-born American writer and philosopher in the early 20th century. A prolific man of letters, Santayana believed in the human capability to learn, regardless of age. He resigned his post at Harvard University after only five years to pursue his own studies, which led him to publish numerous works — from novels to philosophy treatises. His words here remind us that real youth is mental, not physical: In staying curious and lighthearted, we can experience youth at any age.